Process of making zirconia based ceramic material

ABSTRACT

A zirconia based ceramic material having improved and well-balanced mechanical strength and toughness consists essentially of 0.5 to 50 vol % of Al 2  O 3  having an average grain size of 2 μm or less and the balance of a partially stabilized zirconia having an average grain size of 5 μm or less. The partially stabilized zirconia consists essentially of 8 to 12 mol % of CeO 2 , 0.05 to 4 mol % of TiO 2  and the balance of ZrO 2 . Fine Al 2  O 3  grains having an average grain size of 1 μm or less are dispersed within the grains of the partially stabilized zirconia at a dispersion ratio. The dispersion ratio is defined as a ratio of the number of Al 2  O 3  grains dispersed within the grains of the partially stabilized zirconia relative to the number of the entire Al 2  O 3  grains dispersed in the ceramic material, and at least 2% in the present invention. The ceramic material can be made by the following process. A first constituent formed of the partially stabilized zirconia is mixed with a second constituent formed of Al 2  O 3  to obtain a mixture. The mixture is molded into a desired shape, and then sintered in the air under an atmospheric pressure to obtain the ceramic material.

This application is a divisional, of application Ser. No. 08/544,285, filed on Oct. 17, 1995 now U.S. Pat. No. 5,728,636.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a zirconia based ceramic material having excellent mechanical strength and toughness, and also a process of making the same product.

2. Disclosure of the Prior Art

A partially stabilized zirconia, which is obtained by the addition of a preferable amount of a stabilizer such as yttrium oxide (Y₂ O₃), cerium oxide (CeO₂), magnesium oxide (MgO), or the like., to zirconia (ZrO₂), has been well known as a ceramic material having higher mechanical strength and toughness than another popular ceramic materials such as alumina (Al₂ O₃), silicon carbide (SiC), silicon nitride (Si₃ N₄), or the like. The partially stabilized zirconia possesses a unique mechanism for improving the mechanical strength and toughness. That is, a phase transformation from metastable tetragonal zirconia to stable monoclinic zirconia is caused under a stress concentration, for example, at the tip of a crack developed in the partially stabilized zirconia. Since such a transformation, which is usually called as a stress induced phase transformation, is accompanied with a volume expansion of about 4%, a further progress of the crack in the partially stabilized zirconia can be effectively prevented. For example, a partially stabilized zirconia containing Y₂ O₃ as the stabilizer exhibits a mechanical strength of 1000 MPa or more. Particularly, it is already known that a ceramic material comprising Al₂ O₃ grains as a dispersing phase and the partially stabilized zirconia containing Y₂ O₃ as a matrix exhibits a higher mechanical strength of 2400 to 3000 MPa. However, these ceramic materials provide a relatively low mechanical toughness which is in a range of 5 to 6 MPa m^(1/2) and not always enough to use as structural and machine parts. On the other hand, a partially stabilized zirconia containing CeO₂ as the stabilizer exhibits a high mechanical toughness of 10 to 20 MPa m^(1/2). Additionally, the partially stabilized zirconia containing CeO₂ is capable of stably maintaining the metastable tetragonal zirconia at a temperature range of 200° C. to 400° C., and particularly keeping the mechanical strength even under a hydrothermal condition. The partially stabilized zirconia containing Y₂ O₃ exhibits a low temperature degradation of mechanical strength at the temperature range of 200° C. to 400° C. When the partially stabilized zirconia containing Y₂ O₃ is kept at the temperature range for an extended time period, most of tetragonal zirconia included in the partially stabilized zirconia containing Y₂ O₃ is transformed to monoclinic zirconia accompanying with the volume expansion of about 4%. By the volume expansion, micro cracks are developed in the partially stabilized zirconia containing Y₂ O₃, so that the mechanical strength is decreased. In addition, the tetragonal-to monoclinic phase transformation is accelerated under the hydrothermal condition, so that the mechanical strength is considerably decreased. Thus, the partially stabilized zirconia containing CeO₂ exhibits a higher mechanical strength than the partially stabilized zirconia containing Y₂ O₃ at the above temperature range, and particularly under the hydrothermal condition. However, the mechanical strength of the partially stabilized zirconia containing CeO₂ is in a range of 600 to 800 MPa at a room temperature, which is lower than that of the partially stabilized zirconia containing Y₂ O₃. Therefore, by improving mechanical strength of the partially stabilized zirconia containing CeO₂, more expanded application fields will be expected as a material overcoming the low temperature degradation of the mechanical strength of the widely-used partially stabilized zirconia containing Y₂ O₃. In other words, a zirconia based ceramic material having improved and well-balanced mechanical strength and toughness is desired.

Japanese Early Patent Publication KOKAI! No. 5-246760 discloses a zirconia based composite material comprising a matrix of a partially stabilized zirconia containing 5 to 30 mol % of CeO₂ and a secondary phase of at least one selected from Al₂ O₃, SiC, Si₃ N₄, B₄ C, carbides, nitrides and borides of elements of groups IVa, Va, VIa of the periodic table. Fine grains of the secondary phase are dispersed within grains as well as in grain boundaries of the partially stabilized zirconia matrix. When the content of CeO₂ is more than 30 mol %, the mechanical strength of the composite material is lowered due to the increase of cubic zirconia in the matrix. When the content of CeO₂ is less than 5 mol %, a formation of metastable tetragonal zirconia in the matrix is not enough. The composite material contains 0.5 to 50 vol % and more preferably 2.5 to 30 vol % of the secondary phase.

Japanese Early Patent Publication KOKAI! No. 63-144167 discloses the following ceramic materials (a) to (c):

(a) a ceramic material comprising 45-94.75 mol % of ZrO₂, 5-45 mol % of TiO₂ and 0.25-10 mol % of a rare earth oxide selected from the group consisting of Gd₂ O₃, Yb₂ O₃, Nd₂ O₃, Tb₂ O₃, Pr₂ O₃, Dy₂ O₃, Ho₂ O₃, Sm₂ O₃, and Er₂ O₃ ;

(b) a ceramic material comprising 48-94.75 mol % of ZrO₂, 5-45 mol % of TiO₂ and 0.25-7 mol % of Y₂ O₃ ;

(c) a ceramic material comprising 35-93.5 mol % of ZrO₂, 5-45 mol % of TiO₂ and 1.5-20 mol % of CeO₂.

For example, in the ceramic material (a), a grain growth of tetragonal zirconia is prevented by adding a small amount of the rare earth oxide to a partially stabilized zirconia containing TiO₂ as the stabilizer, so that the mechanical properties are improved. In addition, this prior art discloses that grains of cubic zirconia and/or zirconium titanate (ZrTiO₄) are effective to prevent the grain growth of tetragonal zirconia. These ceramic materials (a) to (c) can be used as a dispersing phase to reinforce a ceramic matrix such as alumina, mullite, or the like. For example, a ceramic composite consisting of about 5 vol % of the ceramic material and the balance of the ceramic matrix may be useful as a material improving mechanical properties of the ceramic matrix.

Japanese Patent Publication KOKOKU! No. 64-7029 discloses a ceramic material comprising 61 to 87 wt % of ZrO₂, 11 to 27 wt % of CeO₂ and 20 wt % or less of Al₂ O₃. The ceramic material mainly contains tetragonal zirconia, in which CeO₂ forms a solid solution with ZrO₂. 20% or less of the entire zirconia crystals in the ceramic material is monoclinic zirconia and/or cubic zirconia. This prior art discloses that a sintered body produced by preparing a mixture of ZrO₂, CeO₂ and Al₂ O₃ with the above composition and sintering the mixture exhibits a mechanical strength comparable to a sintered body in ZrO₂ -Y₂ O₃ system. This prior art also indicates that the ceramic material may contains 2 wt % or less of SiO₂, TiO₂ and Fe₂ O₃, etc., as impurities. That is, since a ZrO₂ powder used in this prior art is prepared by treating a zircon sand containing the impurities with an acid to obtain an acid chloride of zirconium, and performing a heat-treatment to the acid chloride, there is a probability of remaining a small amount of the impurities in the ZrO₂ powder. On the other hand, when the content of Al₂ O₃ is more than 20 wt %, a sintering temperature of the ceramic material is raised, so that the grain growth of the zirconia crystals is enhanced. By such a grain growth, variations in mechanical strength of the ceramic material are increased.

Thus, various attempts have been made to improve the mechanical properties of the zirconia based ceramic material.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a zirconia based ceramic material having improved and well-balanced mechanical strength and toughness. That is, the zirconia based ceramic material consists essentially of 0.5 to 50 vol % of Al₂ O₃ grains having an average grain size of 2 μm or less and the balance of grains of a partially stabilized zirconia having an average grain size of 5 μm or less. The partially stabilized zirconia consists essentially of 8 to 12 mol % of CeO₂, 0.05 to 4 mol % of TiO₂ and the balance of ZrO₂. Al₂ O₃ grains having an average grain size of 1 μm or less are dispersed within the grains of the partially stabilized zirconia at a dispersion ratio. The dispersion ratio is defined as a ratio of the number of Al₂ O₃ grains dispersed within the grains of the partially stabilized zirconia relative to the number of the entire Al₂ O₃ grains dispersed in the ceramic material, and at least 2%.

In a preferred embodiment of the present invention, at least 90 vol % of the partially stabilized zirconia is tetragonal zirconia.

A further object of the present invention is to provide a process of making the zirconia based ceramic material. That is, a first constituent formed of the partially stabilized zirconia consisting essentially of 8 to 12 mol % of CeO₂, 0.05 to 4 mol % of TiO₂ and the balance of ZrO₂ is mixed with a second constituent formed of Al₂ O₃ such that the ceramic material essentially consists of 0.5 to 50 vol % of Al₂ O₃ and the balance of the partially stabilized zirconia. A resultant mixture is molded into a desired shape to form a green compact thereof. The green compact is sintered in an oxidizing atmosphere under an atmospheric pressure to obtain the ceramic material.

It is preferred that after the sintering step, a hot isostatic pressing (HIP) treatment is performed to the ceramic material in an oxygen containing atmosphere to reach a higher relative density of the ceramic material.

In a preferred embodiment of the present invention, a powder of the partially stabilized zirconia as the first constituent can be prepared by the sub-steps of mixing a first aqueous solution of a zirconium salt with a second aqueous solution including cerium and titanium salts, hydrolyzing the mixed solution with an alkali aqueous solution to obtain a precipitate, drying and heating the precipitate to obtain a calcined powder, and pulverizing the calcined powder. On the other hand, a powder of α-Al₂ O₃ having an average grain size of 0.5 μm or less or a powder of γ-Al₂ O₃ having a specific surface area of 100 m² /g or more can be used as the second constituent.

These and still other objects and advantages features of the present invention will become more apparent from the following description and examples of the present invention when taken in conjunction with the attached drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a transmission electron micrograph of a zirconia based ceramic material of Example 20 of the present invention.

DETAIL DESCRIPTION OF THE INVENTION

A zirconia based ceramic material of the present invention consists essentially of 0.5 to 50 vol % of Al₂ O₃ grains having an average grain size of 2 μm or less and the balance of grains of a partially stabilized zirconia having an average grain size of 5 μm or less. The partially stabilized zirconia consists essentially of 8 to 12 mol % of CeO₂, 0.05 to 4 mol % of TiO₂ and the balance of ZrO₂. When the content of CeO₂ is less than 8 mol %, a stabilization of tetragonal zirconia is not enough, and a formation of monoclinic zirconia is enhanced so that micro cracks will develop in the ceramic material. When the content of CeO₂ is more than 12 mol %, the tetragonal zirconia is excessively stabilized, so that the mechanical toughness of the ceramic material can not be sufficiently improved by a mechanism of a stress induced phase transformation from tetragonal zirconia to monoclinic zirconia. In a ZrO₂ -TiO₂ phase diagram, it is known that the tetragonal zirconia forms a solid solution with about 18 mol % or less of TiO₂ at a high temperature. TiO₂ is capable of keeping the tetragonal zirconia metastably at a room temperature as well as Y₂ O₃ and CeO₂, and enhancing a grain growth of ZrO₂. Therefore, when an excess amount of TiO₂ is used as a stabilizer of the tetragonal zirconia, the mechanical strength of the ceramic material would be lowered due to the occurrence of an abnormal grain growth of ZrO₂. The ceramic material of the present invention has a so called nano-composite structure which is useful to improve the mechanical properties of the ceramic material. That is, fine Al₂ O₃ grains having an average grain size of 1 μm or less are dispersed within the grains of the partially stabilized zirconia at a dispersion ratio. The dispersion ratio is defined as a ratio of the number of Al₂ O₃ grains dispersed within the grains of the partially stabilized zirconia relative to the number of the entire Al₂ O₃ grains dispersed in the ceramic material. In the present invention, the dispersion ratio is at least 2%. Therefore, a controlled grain growth of the grains of the partially stabilized zirconia is required to form the nano-composite structure. When the content of TiO₂ is less than 0.05 mol %, a required grain growth of the partially stabilized zirconia for the nano-composite structure is not achieved. When the content of TiO₂ is more than 4 mol %, the abnormal grain growth of the partially stabilized zirconia happens, so that it will act as a fracture origin of the ceramic material. Therefore, only when both of the TiO₂ and CeO₂ contents are in the present ranges, it is possible to provide the nano-composite structure of the ceramic material while enhancing the stabilization of tetragonal zirconia without causing the abnormal grain growth of the partially stabilized zirconia. In addition, only when both of the TiO₂ and CeO₂ contents are in the present ranges, it is possible to obtain an adequate critical stress necessary for causing the stress induced phase transformation to thereby improve the mechanical strength and toughness of the ceramic material. In this case, it is preferred that the partially stabilized zirconia contains at least 90 vol % of the tetragonal zirconia and 10 vol % or less of the monoclinic zirconia.

In the present invention, the Al₂ O₃ content in the ceramic material is in a range of 0.5 to 50 vol %, and more preferably 20 to 40 vol %. When the Al₂ O₃ content is less than 0.5 vol %, the formation of the nano-composite structure is not enough. As the Al₂ O₃ content is increased more than 50 vol %, an improvement of mechanical toughness of the ceramic material by the partially stabilized zirconia is prevented.

The nano-composite structure of the present ceramic material is explained in detail as below. A residual stress field is formed around each of the Al₂ O₃ grains dispersed within the grains of the partially stabilized zirconia by a mismatch of thermal expansion coefficient between the partially stabilized zirconia and Al₂ O₃, so that the grains of the partially stabilized zirconia are considerably reinforced. That is, a large number of dislocations are generated within the grains of the partially stabilized zirconia by the residual stress field. The dislocations are piled up each other to form sub-grain boundaries within the grains of the partially stabilized zirconia. The formation of sub-grain boundaries provides a fine grain structure to the partially stabilized zirconia and is useful to increase the critical stress necessary for causing the stress induced phase transformation. In addition, since cracks developed in the ceramic material are bowed or deflected by the Al₂ O₃ grains uniformly dispersed in the grain boundaries of the partially stabilized zirconia, further progresses of the cracks would be effectively prevented to improve the fracture toughness of the ceramic material. These advantages of the nano-composite structure of the ceramic material are efficiently obtained when the ceramic material is formed with the Al₂ O₃ grains having the average grain size of 2 μm or less and the grains of the partially stabilized zirconia having the average grain size of 5 μm or less, and also the Al₂ O₃ grains having an average grain size of 1 μm or less are dispersed within the grains of the partially stabilized zirconia at the dispersion ratio of at least 2%. Furthermore, it is preferred that the grains of the partially stabilized zirconia having an average grain size of 1 μm or less are partly dispersed within grains of Al₂ O₃, so that the Al₂ O₃ grains are reinforced by the residual stress field resulting from the mismatch of thermal expansion coefficient and the formation of sub-grain boundaries therein.

A process of making the ceramic material of the present invention is explained in detail. A first constituent formed of the partially stabilized zirconia consisting essentially of 8 to 12 mol % of CeO₂ and 0.05 to 4 mol % of TiO₂ and the balance of ZrO₂ is mixed with a second constituent formed of Al₂ O₃ such that the ceramic material essentially consists of 0.5 to 50 vol % of Al₂ O₃ and the balance of the partially stabilized zirconia. A resultant mixture is molded into a desired shape to form a green compact thereof. The green compact is sintered in an oxidizing atmosphere under an atmospheric pressure to obtain the ceramic material. In particular, when the sintering step is carried out to obtain the ceramic material having a relative density of 95% or more, it is preferred that after the sintering step, a hot-isostatic pressing (HIP) treatment is performed to the ceramic material in an oxygen gas containing atmosphere to remove residual pores and further improve the mechanical strength and toughness of the ceramic material. For example, a mixture gas of oxygen gas and a rare gas such as argon may be used as the oxygen gas containing atmosphere. In this case, it is preferred that the mixture gas contains 5 vol % or more of the oxygen gas.

The first constituent can be prepared by any one of the following sub-processes 1! to 3!.

In the sub-process 1!, a powder of a first partially stabilized zirconia having an average grain size of 0.5 μm or less and containing only CeO₂ is mixed with a TiO₂ powder having an average grain size of 0.5 μm or less. For example, these powders may be ball-milled in the presence of a solvent such as ethanol, acetone or toluene, or the like, to obtain the mixed powder. The mixed powder is heated at a temperature of 800° to 1000° C. to obtain a calcined powder. At the temperature, TiO₂ forms a solid solution with ZrO₂ to stabilize tetragonal zirconia. The calcined powder is pulverized to obtain a powder of the partially stabilized zirconia as the first constituent.

In the sub-process 2!, an aqueous solution of a zirconium salt is mixed with an aqueous solution including cerium and titanium salts to obtain a mixed aqueous solution. The mixed solution is hydrolyzed by adding an alkali aqueous solution thereto, to thereby generate a precipitate. The precipitate is dried and heated to obtain a first calcined powder. For example, the precipitate may be heated at a temperature of about 800° C. in the air. Then, the calcined powder is pulverized to obtain a powder of the partially stabilized zirconia as the first constituent. For example, the calcined powder may be ball-milled in the presence of a solvent.

In the sub-process 3!, an aqueous solution including zirconium and cerium salts is mix ed with an organic solution of an alkoxide of titanium to obtain a mixed solution. The mixed solution is hydrolyzed by adding an alkali aqueous solution thereto, to thereby generate a precipitate. To obtain a powder of the partially stabilized zirconia as the first constituent from the precipitate, the same procedure as the sub-process 2! may be adopted.

It is preferred to use as the second constituent a powder of α-Al₂ O₃ having an average grain size of 0.5 μm or less or a powder of γ-Al₂ O₃ having a specific surface area of 100 m² /g or more. In addition, the second constituent can be prepared by any one of the following sub-processes 4! and 5!.

In the sub-process 4!, an aqueous solution of aluminum salt is hydrolyzed by adding an alkali aqueous solution such as aqueous ammonia thereto, to thereby generate a precipitate. The precipitate is dried and heated at a temperature of about 800° C. for several hours in the air to obtain an Al₂ O₃ powder as the second constituent.

In the sub-process 5!, an organic solution of an alkoxide of aluminum is hydrolyzed to obtain a precipitate. The precipitate is dried and heated at a temperature of about 800° C. for several hours in the air to obtain an Al₂ O₃ powder as the second constituent.

The mixture of the first and second constituents can be prepared by any one of the following sub-processes 6! to 8!.

In the sub-process 6!, a powder of the partially stabilized zirconia containing CeO₂ and TiO₂ as the first constituent is mixed with an aqueous solution of an aluminum salt as the second constituent to obtain a mixed solution. The mixed solution is hydrolyzed by adding an alkali aqueous solution such as aqueous ammonia thereto, to thereby generate a precipitate.

The precipitate is dried and heated to obtain a mixed powder of Al₂ O₃ and the partially stabilized zirconia as the mixture of the first and second constituents. For example, the precipitate may be heated at a temperature of about 800° C. for several hours.

In the sub-process 7!, a powder of the partially stabilized zirconia containing CeO₂ and TiO₂ as the first constituent is mixed with an organic solution of an alkoxide of aluminum as the second constituent to obtain a mixed solution. The mixed solution is hydrolyzed to generate a precipitate.

To obtain a mixed powder of Al₂ O₃ and the partially stabilized zirconia as the mixture of the first and second constituents from the precipitate, the same procedure as the sub-process 6! may be adopted.

In each of the sub-processes 6! and 7!, it is preferred to use as the first constituent the powder of the partially stabilized zirconia prepared by any one of the above sub-processes 1! to 3!.

In the sub-process 8!, a first aqueous solution of zirconium is hydrolyzed by an alkali aqueous solution to prepare a sol solution of zirconia. The sol solution is mixed with a second aqueous solution including cerium and titanium salts and a third aqueous solution of an aluminum salt as the second constituent to obtain a mixed aqueous solution. The mixed solution is hydrolyzed by adding an alkali aqueous solution thereto, to thereby generate a precipitate. The precipitate is dried and heated to obtain a mixed powder of Al₂ O₃ and the partially stabilized zirconia as the mixture of the first and second constituents. For example, the precipitate may be heated at a temperature of about 800° C. for several hours.

In case of using the powder of the partially stabilized zirconia prepared by the sub-process 2! or 3! as the first constituent, the powder of γ-Al₂ O₃ having a specific surface area of 100 m² /g or more as the second constituent, or the mixed powder prepared by any one of the sub-processes 6! to 8! as the mixture of the first and second constituents, there is a probability of causing inconvenience to the molding and sintering steps because each of those powders has an extremely large specific surface area, in other words, exhibits a poor compactibility. In particular, it would be difficult to mold such a powder into a desired shape by a regular dry pressing or injection molding method. Therefore, it is preferred that a mixed powder of the powder of the partially stabilized zirconia prepared by the sub-process 2! or 3! and an Al₂ O₃ powder as the second constituent is heated at a temperature of 1000° C. or more and less than a sintering temperature thereof to obtain a second calcined powder, and then pulverized to control a particle size of the second calcined powder prior to the molding and sintering steps. Similarly, it is preferred that a mixed powder of a powder of the partially stabilized zirconia as the first constituent and the powder of γ-Al₂ O₃ having a specific surface area of 100 m² /g or more is heated at a temperature of 1000° C. or more and less than a sintering temperature thereof to obtain a calcined powder, and then pulverized to control a particle size of the calcined powder prior to the molding and sintering steps. In addition, it is preferred that the mixed powder prepared by any one of the sub-processes 6! to 8! is heated at a temperature of 1000° C. or more and less than a sintering temperature thereof to obtain a calcined powder, and then pulverized to control a particle size of the calcined powder prior to the molding and sintering steps. For example, the calcined powder may be ball-milled in the presence of a solvent.

EXAMPLES 1 to 17

A zirconia based ceramic material of Example 1 was produced by the following process. A powder of a first partially stabilized zirconia containing CeO₂ and having an average grain size of 0.3 μm and a TiO₂ powder having an average grain size of 0.3 μm were ball-milled in the presence of ethanol for 24 hours by the use of balls made of a partially stabilized zirconia and a polyethylene vessel, and then dried to obtain a first mixed powder. The first mixed powder was heated at 800° C. in the air for 3 hours to obtain a calcined powder of a second partially stabilized zirconia containing CeO₂ and TiO₂. As listed in Table 1, the second partially stabilized zirconia of Example 1 contains 8 mol % of CeO₂ and 1 mol % of TiO₂. The calcined powder and a powder of α-Al₂ O₃ having an average grain size of 0.2 μm were ball-milled in the presence of ethanol for 24 hours by the use of the zirconia balls and the polyethylene vessel, and then dried to obtain a second mixed powder. As listed in Table 1, an additive amount of the α-Al₂ O₃ powder was determined such that the ceramic material of Example 1 contains 30 vol % of Al₂ O₃. The second mixed powder was molded into a disk having the diameter of 60 mm and the thickness of 5 mm by means of a one-axis press molding and cold isostatic pressing (CIP) treatment. The disk was sintered at 1500° C. in the air for 2 hours under an atmospheric pressure to obtain a disk-shaped sintered product of the zirconia based ceramic material of Example 1. Each of ceramic materials of Examples 2 to 17 was made in accordance with the same process as Example 1 except for different CeO₂ and TiO₂ contents, as listed in Table 1.

Each of the sintered products of Examples 1 to 17 reaches a relative density of 99% or more. It was observed by the use of scanning electron microscope and transmission electron microscope that as the TiO₂ content is greater, average grain sizes of the partially stabilized zirconia and Al₂ O₃ are increased without depending on the CeO₂ content. The average grain size of the partially stabilized zirconia is in a range of 0.8 to 4.8 μm. The average grain size of Al₂ O₃ is less that 2 μm. It was also observed that fine grains of Al₂ O₃ having an average grain size of 1 μm or less are dispersed within grains of the partially stabilized zirconia at a dispersion ratio listed in Table 1, and also fine grains of the partially stabilized zirconia having an average grain size of 1 μm or less are partly dispersed within relatively large Al₂ O₃ grains. To estimate mechanical strength of the ceramic material, a three point bending strength was measured according to the test method of JIS (Japanese Industrial Standard) R1601. For measuring the bending strength, specimens of 4×3×40 mm of the ceramic material were formed by cutting, grinding and polishing the disk-shaped sintered product. In addition, fracture toughness of the ceramic material was measured in accordance with IF method. The test results are listed in Table 1. Furthermore, the identification of crystal phases of the partially stabilized zirconia and quantification of the crystal phases were carried out by means of a X-ray diffraction analysis. In Table 1, "T", "C" and "M" designate tetragonal zirconia, cubic zirconia and monoclinic zirconia, respectively. The partially stabilized zirconia of each of Examples 1 and 2 contains at least 90 vol % of the tetragonal zirconia and less than 10 vol % of the monoclinic zirconia.

The partially stabilized zirconia of each of Examples 3 to 17 substantially consists of the tetragonal zirconia. To determine a dispersion ratio (W), which is defined as a ratio of the number of the Al₂ O₃ grains dispersed within the grains of the partially stabilized zirconia relative to the number of the entire Al₂ O₃ grains dispersed in the ceramic material, the number (S) of the entire Al₂ O₃ grains dispersed at an observation area in the ceramic material and the number (n) of the Al₂ O₃ grains dispersed within the grains of the partially stabilized zirconia at the observation area were counted by using the transmission electron microscope (TEM) or scanning electron microscope (SEM). The dispersion ratio (W) is represented by substituting the counted numbers (S) and (n) into the following equation: W(%)=(n/S)×100. Results are listed in Table 1.

COMPARATIVE EXAMPLES 1 to 3

Each of zirconia based ceramic materials of Comparative Examples 1 to 3 was produced in accordance with the same process as Example 1 except for different contents of CeO₂ and TiO₂, as listed in Table 2. SEM and TEM observations, the measurements of bending strength and fracture toughness, and X-ray diffraction analysis were performed to the respective ceramic materials of Comparative Examples 1 to 3. Results are also listed in Table 2.

Since large cracks developed in the ceramic material of Comparative Example 1 after the sintering step, the measurements of the mechanical properties were not performed. By the X-ray diffraction analysis, it was identified that a partially stabilized zirconia of the ceramic material of Comparative Example 1 contains about 80 vol % of monoclinic zirconia. The ceramic material of Comparative Example 2 was densely sintered. It was observed by the SEM and TEM observations that average grain sizes of a partially stabilized zirconia and Al₂ O₃ of the ceramic material of Comparative Example 2 are about 0.5 μm. In addition, it was observed that most of Al₂ O₃ grains are dispersed in grain boundaries of the partially stabilized zirconia, and a dispersion ratio of the Al₂ O₃ grains dispersed within the grains of the partially stabilized zirconia is 0.8%. Since TiO₂ was not used in Comparative Example 2, the bending strength of the ceramic material was only 520 MPa. It was identified by the X-ray diffraction analysis that the partially stabilized zirconia of Comparative Example 2 contains about 20 vol % of monoclinic zirconia. On the other hand, it was observed that the ceramic material of Comparative Example 3 comprises Al₂ O₃ grains dispersed within grains of a partially stabilized zirconia, abnormally grown ZrO₂ grains of about 15 to 20 μm, and residual pores left within the grains and at triple points of the partially stabilized zirconia. Since the partially stabilized zirconia of Comparative Example 3 contains an excess amount of TiO₂, i.e., 8 mol %, the ceramic material exhibited low bending strength and fracture toughness, as listed in Table 2. It was identified by the X-ray diffraction analysis that the partially stabilized zirconia of Comparative Example 3 contains a small amount of cubic zirconia.

                                      TABLE 1     __________________________________________________________________________                       Average                            Average     Al.sub.2 O.sub.3               CeO.sub.2                   TiO.sub.2                       Grain                            Grain Crystal                                      Bending                                           Fracture                                                 Dispersion     Content   Content                   Content                       Size of                            Size of                                  phase                                      Strength                                           Toughness                                                 Ratio of     (vol %)   (mol %)                   (mol %)                       PSZ(μm)                            Al.sub.2 O.sub.3 (μm)                                  of PSZ                                      (MPa)                                           (MPa · m.sup.1/2)                                                 α-Al.sub.2 O.sub.3     __________________________________________________________________________                                                 (%)     Example 1           30  8   1   2.2  1.0   T + M                                      720  21.5  3.1     Example 2           30  8   2   2.8  1.1   T + M                                      790  20.1  3.4     Example 3           30  8   3   3.7  1.3   T   820  18.0  3.8     Example 4           30  8   4   4.8  1.8   T   600  13.9  4.4     Example 5           30  10  0.2 1.1  0.5   T   1140 17.5  2.7     Example 6           30  10  0.5 1.6  0.6   T   1120 16.3  3.0     Example 7           30  10  1   2.1  0.7   T   1090 15.7  3.3     Example 8           30  10  2   2.7  0.9   T   1060 13.9  3.7     Example 9           30  10  3   3.6  1.1   T   960  11.8  3.9     Example 10           30  10  4   4.6  1.6   T   830  10.5  4.5     Example 11           30  12  0.05                       0.8  0.5   T   1000 11.5  2.5     Example 12           30  12  0.1 1.0  0.6   T   1100 11.0  2.8     Example 13           30  12  0.5 1.5  0.6   T   1010 9.6   3.2     Example 14           30  12  1   2.0  0.7   T   970  8.8   3.4     Example 15           30  12  2   2.6  0.8   T   950  7.8   3.9     Example 16           30  12  3   3.5  1.0   T   800  7.1   4.1     Example 17           30  12  4   4.3  1.5   T   670  6.6   4.6     __________________________________________________________________________      *PSZ = Partially Stabilized Zirconia

                                      TABLE 2     __________________________________________________________________________                       Average                            Average     Al.sub.2 O.sub.3               CeO.sub.2                   TiO.sub.2                       Grain                            Grain Crystal                                      Bending                                           Fracture                                                 Dispersion     Content   Content                   Content                       Size of                            Size of                                  phase of                                      Strength                                           Toughness                                                 Ratio of     (vol %)   (mol %)                   (mol %)                       PSZ(μm)                            Al.sub.2 O.sub.3 (μm)                                  of PSZ                                      (MPa)                                           (MPa · m.sup.1/2)                                                 α-Al.sub.2 O.sub.3     __________________________________________________________________________                                                 (%)     Comparative           30  6   1   --   --    T + M                                      --   --    --     Example 1     Comparative           30  8   0   0.5  0.5   T + M                                      520  22.1  0.8     Example 2     Comparative           30  12  8   10   3.5   T + C                                      340  4.1   5.5     Example 3     __________________________________________________________________________      *PSZ = Partially Stabilized Zirconia

EXAMPLES 18 to 22 AND COMPARATIVE EXAMPLES 4 AND 5

A zirconia based ceramic material of Example 18 was produced by the following process. A powder of a first partially stabilized zirconia containing 10 mol % of CeO₂ and a TiO₂ powder were ball-milled in the presence of ethanol for 24 hours by the use of balls made of a partially stabilized zirconia and a polyethylene vessel, and then dried to obtain a first mixed powder. An additive amount of the TiO₂ powder was determined such that a partially stabilized zirconia in the ceramic material of Example 18 contains 1 mol % of TiO₂. The first mixed powder was heated at 800° C. in the air for 3 hours to obtain a calcined powder of the partially stabilized zirconia containing 10 mol % of CeO₂ and 1 mol % of TiO₂. The calcined powder and a 99.9% or more pure α-Al₂ O₃ powder having an average grain size of 0.2 μm were ball-milled in the presence of ethanol for 24 hours by the use of the zirconia balls and the polyethylene vessel, and then dried to obtain a second mixed powder. As listed in Table 3, an additive amount of the Al₂ O₃ powder was determined such that the ceramic material of Example 18 contains 10 vol % of Al₂ O₃. The second mixed powder was molded into a disk having the diameter of 60 mm and the thickness of 5 mm by means of a one-axis press molding method and cold isostatic pressing (CIP) treatment. The disk was sintered at 1500° C. in the air under an atmospheric pressure for 2 hours to obtain a disk-shaped sintered product of the zirconia based ceramic material of Example 18. Each of ceramic materials of Examples 19 to 22 and Comparative Examples 4 and 5 was produced in accordance with the same process as Example 18 except for different content of Al₂ O₃, as listed in Table 3.

Each of the ceramic materials of Examples 18 to 22 and Comparative Examples 4 and 5 reaches a relative density of 99% or more. As the content of Al₂ O₃ is increased, it was observed that a grain growth of the partially stabilized zirconia is prevented more efficiently. In the ceramic material of Comparative Example 4 in which the Al₂ O₃ powder is not added, it was observed that an average grain size of the partially stabilize zirconia is about 5 μm. As the Al₂ O₃ content in the ceramic material is increased, it was observed that the average grain size of the partially stabilized zirconia is decreased. In Examples 18 to 22, the average grain size of the partially stabilized zirconia is in a range of 1.2 to 3.0 μm. Additionally, in the ceramic materials containing 30 vol % or more of Al₂ O₃, it was observed that a sintering between Al₂ O₃ grains is enhanced, and a large number of Al₂ O₃ grains are dispersed in grain boundaries of the partially stabilized zirconia. As shown in FIG. 1, a transmission electron micrograph of the ceramic material of Example 20 indicates that the ceramic material has a so called mutual nano-composite structure, in which fine Al₂ O₃ grains are partly dispersed within grains of the partially stabilized zirconia and also fine grains of the partially stabilized zirconia are partly dispersed within Al₂ O₃ grains having the grain size of about 1 μm. The Al₂ O₃ grains having the grain size of about 1 μm are dispersed in the grain boundaries of the partially stabilized zirconia. By X-ray diffraction analysis, it was identified that the partially stabilized zirconia of each of the ceramic materials of the Examples 18 to 22 and Comparative Examples 4 and 5 contains at least 95 vol % of tetragonal zirconia and 5 vol % or less of monoclinic zirconia. To estimate mechanical strength of the ceramic material, a three point bending strength was measured according to the test method of JIS (Japanese Industrial Standard) R1601. For measuring the bending strength, specimens of 4×3×40 mm of the ceramic material were formed by cutting, grinding and polishing the disk-shaped sintered product. In addition, fracture toughness and Vickers hardness of the ceramic material were measured. The test results are listed in Table 3. To determine a dispersion ratio (W), which is defined as a ratio of the number of the Al₂ O₃ grains dispersed within the grains of the partially stabilized zirconia relative to the number of the entire Al₂ O₃ grains dispersed in the ceramic material, the number (S) of the entire Al₂ O₃ grains dispersed at an observation area in the ceramic material and the number (n) of the Al₂ O₃ grains dispersed within the grains of the partially stabilized zirconia at the observation area were counted by using the transmission electron microscope (TEM) or by observing a polished and heat-treated surface of the sintered product with the use of scanning electron microscope (SEM). The dispersion ratio (W) is represented by substituting the counted numbers (S) and (n) into the following equation: W(%)=(n/S)×100. Results are also listed in Table 3.

EXAMPLE 23

A zirconia based ceramic material of Examples 23 was produced in accordance with the same process as Example 20 except for the sintering temperature is 1450° C. In this Example, after the sintering step, a hot isostatic pressing (HIP) treatment was performed to the ceramic material for 1 hour at the temperature of 1350° C. under the pressure of 150 MPa of a mixture gas consisting of 10 vol % of oxygen and the balance of argon. The ceramic material was densified without leaving residual pores therein by the HIP treatment to reach a theoretical density thereof. By TEM and SEM observations of the ceramic material, it was observed that the ceramic material has a so called mutual nano-composite structure, in which fine Al₂ O₃ grains are dispersed within grains of the partially stabilized zirconia and also fine grains of the partially stabilized zirconia are dispersed within relatively large Al₂ O₃ grains. No grain growth caused by the HIP treatment was observed. Average grain sizes of the partially stabilized zirconia and Al₂ O₃ of this ceramic material are 1.5 μm and 0.5 μm, respectively. By X-ray diffraction analysis, it was identified that the partially stabilized zirconia contains at least 99 vol % of tetragonal zirconia. Measurements of bending strength, Vickers hardness and fracture toughness of the ceramic material were performed according to the same procedures as Example 1. Results are listed in Table 4.

COMPARATIVE EXAMPLE 6

A zirconia based ceramic material of Comparative Example 6 was produced in accordance with the same process as Example 20 except for using a 99.9% or more pure α-Al₂ O₃ powder having the average grain size of 3 μm. A relative density of the ceramic material reaches 99% or more. By TEM and SEM observations of the ceramic material, it was observed that most of Al₂ O₃ grains are dispersed in grain boundaries of a partially stabilized zirconia. In addition, it was identified by X-ray diffraction analysis that the partially stabilized zirconia contains at least 95 vol % of tetragonal zirconia. Measurements of bending strength, Vickers hardness and fracture toughness of the ceramic material were performed according to the same procedures as Example 1. Results are listed in Table 4.

                                      TABLE 3     __________________________________________________________________________                       Average                            Average     Al.sub.2 O.sub.3               CeO.sub.2                   TiO.sub.2                       Grain                            Grain Vickers                                       Bending                                            Fracture                                                  Dispersion     Content   Content                   Content                       Size of                            Size of                                  Hardness                                       Strength                                            Toughness                                                  Ratio of     (vol %)   (mol %)                   (mol %)                       PSZ(μm)                            Al.sub.2 O.sub.3 (μm)                                  (GPa)                                       (MPa)                                            (MPa · m.sup.1/2)                                                  α-Al.sub.2 O.sub.3     __________________________________________________________________________                                                  (%)     Comparative           0   10  1   5.0  --    7.5  460  24.0  --     Example 4     Example 18           10  10  1   3.0  0.5   9.1  790  22.3  2.2     Example 19           20  10  1   2.5  0.6   10.3 1020 18.8  2.8     Example 20           30  10  1   2.1  0.7   11.2 1090 15.7  3..3     Example 21           40  10  1   1.5  0.7   12.2 1070 11.9  2.6     Bxample 22           50  10  1   1.2  0.8   13.4 900  7.6   2.0     Comparative           60  10  1   1.0  1.0   14.4 510  4.4   0.9     Example 5     __________________________________________________________________________      *PSZ = Partially Stabilized Zirconia

                                      TABLE 4     __________________________________________________________________________                       Average                            Average     Al.sub.2 O.sub.3               CeO.sub.2                   TiO.sub.2                       Grain                            Grain Vickers                                       Bending                                            Fracture                                                  Dispersion     Content   Content                   Content                       Size of                            Size of                                  Hardness                                       Strength                                            Toughness                                                  Ratio of     (vol %)   (mol %)                   (mol %)                       PSZ(μm)                            Al.sub.2 O.sub.3 (μm)                                  (GPa)                                       (MPa)                                            (MPa · m.sup.1/2)                                                  α-Al.sub.2 O.sub.3     __________________________________________________________________________                                                  (%)     Example 23           30  10  1   1.5  0.5   11.7 1450 16.3  3.4     Comparative           30  10  1   2.3  3.5   10.6 720  16.6  0.3     Bxample 6     __________________________________________________________________________      *PSZ = Partially Stabilized Zirconia

EXAMPLE 24

A zirconia based ceramic material of Example 24 was produced by the following process. A first aqueous solution of zirconium oxychloride (ZrOCl₂.8H₂ O) was hydrolyzed by adding aqueous ammonia thereto, to prepare a sol solution of zirconia. The sol solution was mixed with a second aqueous solution of cerium chloride (CeCl₃.7H₂ O) and a third aqueous solution of titanium chloride (TiCl₄), while agitating a resultant mixture. The mixture was dropped into aqueous ammonia, while agitating the aqueous ammonia, to thereby obtain a precipitate. The concentrations of the second and third aqueous solutions were determined such that a partially stabilized zirconia of the ceramic material contains 11 mol % of CeO₂ and 1 mol % of TiO₂. After the precipitate was washed with water and then dried, it was heated at 850° C. in the air for 3 hours to obtain a calcined powder of the partially stabilized zirconia. The calcined powder and a γ-Al₂ O₃ powder having a specific surface area of 300 m² /g were ball-milled in the presence of ethanol for 24 hours by the use of balls made of a partially stabilized zirconia and a polyethylene vessel, and then dried to obtain a mixed powder. An additive amount of the γ-Al₂ O₃ powder was determined such that the ceramic material contains 30 vol % of Al₂ O₃. The mixed powder was molded into a disk having the diameter of 60 mm and the thickness of 5 mm by means of a one-axis press molding and cold isostatic pressing (CIP) treatment. The disk was sintered at 1500° C. in the air under an atmospheric pressure for 2 hours to obtain a disk-shaped sintered product of the zirconia based ceramic material.

A relative density of this sintered product reaches 99% or more. By SEM and TEM observations, it was observed that a relatively large number of Al₂ O₃ grains are dispersed within grains of the partially stabilized zirconia. In addition, by X-ray diffraction analysis, it was identified that the partially stabilized zirconia contains at least 95 vol % of tetragonal zirconia and less than 5 vol % of monoclinic zirconia. The γ-Al₂ O₃ powder in the mixed powder were completely converted into α-Al₂ O₃ during the sintering step. Measurements of bending strength, Vickers hardness and fracture toughness of the ceramic material were carried out according to the same procedure as Example 1. Results are listed in Table 5.

EXAMPLE 25

A zirconia based ceramic material of Example 25 was produced by the following process. A powder of a partially stabilized zirconia, which is the same as the calcined powder prepared in Example 24, was mixed with a hydrochloride solution of aluminum chloride (AlCl₃), while agitating a resultant mixture. The concentration of the hydrochloride solution was determined such that the ceramic material contains 30 vol % of Al₂ O₃. The mixture was hydrolyzed by an aqueous solution of sodium hydroxide (NaOH) to generate a precipitate. After the precipitate was washed with water and then dried, it was heated at 1000° C. in the air for 3 hours to obtain a mixed powder. The mixed powder was molded into a disk having the diameter of 60 mm and the thickness of 5 mm by means of a one-axis press molding and cold isostatic pressing (CIP) treatment. The disk was sintered at 1500° C. in the air under an atmospheric pressure for 2 hours to obtain a disk-shaped sintered product of the zirconia based ceramic material.

A relative density of this sintered product reaches 99% or more. By SEM and TEM observations, it was observed that a relatively large number of Al₂ O₃ grains are dispersed within grains of the partially stabilized zirconia. In addition, by X-ray diffraction analysis of the ceramic material, it was identified that the partially stabilized zirconia contains at least 95 vol % of tetragonal zirconia and less than 5 vol % of monoclinic zirconia. Measurements of bending strength, Vickers hardness and fracture toughness of the ceramic material were carried out according to the same procedure as Example 1. Results are listed in Table 5.

EXAMPLE 26

A zirconia based ceramic material of Example 26 was produced by the following process. A first aqueous solution of zirconium oxychloride (ZrOCl₂.8H₂ O) was hydrolyzed by adding aqueous ammonia thereto, to prepare a sol solution of zirconia. The sol solution was mixed with a second aqueous solution of cerium chloride (CeCl₃.7H₂ O) and a first isopropanol solution of titanium isopropoxide Ti(iOC₃ H₇)₄ !, while agitating a resultant mixture. The mixture was dropped into aqueous ammonia, while agitating the aqueous ammonia, to thereby obtain a first precipitate. The concentrations of the second aqueous solution and first isopropanol solution were determined such that a partially stabilized zirconia of the ceramic material contains 11 mol % of CeO₂ and 1 mol % of TiO₂. After the first precipitate was washed with water and then dried, it was heated at 850° C. in the air for 3 hours to obtain a calcined powder of the partially stabilized zirconia. The calcined powder was mixed with a second isopropanol solution of aluminum isopropoxide Al(iOC₃ H₇)₃ ! to obtain a mixed solution. The concentration of the second isopropanol solution was determined such that the ceramic material contains 30 vol % of Al₂ O₃. The mixed solution was hydrolyzed to obtain a second precipitate. After the second precipitate is washed with water and then dried, it was heated at 1000° C. in the air for 3 hours to obtain a mixed powder. The mixed powder was molded into a disk having the diameter of 60 mm and the thickness of 5 mm by means of a one-axis press molding and cold isostatic pressing (CIP) treatment. The disk was sintered at 1500° C. in the air for 2 hours under an atmospheric pressure to obtain a disk-shaped sintered product of the zirconia based ceramic material.

A relative density of this sintered product reaches 99% or more. By SEM and TEM observations, it was observed that a relatively large number of Al₂ O₃ grains are dispersed within grains of the partially stabilized zirconia. In addition, by X-ray diffraction analysis of the ceramic material, it was identified that the partially stabilized zirconia contains at least 95 vol % of tetragonal zirconia and less than 5 vol % of monoclinic zirconia. Measurements of bending strength, Vickers hardness and fracture toughness of the ceramic material were carried out according to the same procedure as Example 1. Results are listed in Table 5.

EXAMPLE 27

A zirconia based ceramic material of Example 27 was produced by the following process. A first aqueous solution of zirconium oxychloride (ZrOCl₂.8H₂ O) was hydrolyzed by adding aqueous ammonia thereto, to prepare a sol solution of zirconia. The sol solution was mixed with a second aqueous solution of cerium chloride (CeCl₃.7H₂ O), a third aqueous solution of titanium chloride (TiCl₄), and a fourth aqueous solution of aluminum nitrate Al(NO₃)₃.9H₂ O!, while agitating a resultant mixture. The concentrations of the second and third aqueous solutions were determined such that a partially stabilized zirconia of the ceramic material contains 11 mol % of CeO₂ and 1 mol % of TiO₂. The concentration of the fourth aqueous solution was determined such that the ceramic material contains 30 vol % of Al₂ O₃. The mixture was dropped into aqueous ammonia, while agitating the aqueous ammonia, to thereby obtain a precipitate. After the precipitate was washed with water and then dried, it was heated at 1000° C. in the air for 3 hours to obtain a mixed powder. The mixed powder was molded into a disk having the diameter of 60 mm and the thickness of 5 mm by means of a one-axis press molding and cold isostatic pressing (CIP) treatment. The disk was sintered at 1500° C. in the air for 2 hours under an atmospheric pressure to obtain a disk-shaped sintered product of the zirconia based ceramic material.

A relative density of this sintered product reaches 99% or more. By SEM and TEM observations, it was observed that a relatively large number of Al₂ O₃ grains are dispersed within grains of the partially stabilized zirconia. In addition, by X-ray diffraction analysis of the ceramic material, it was identified that the partially stabilized zirconia contains at least 95 vol % of tetragonal zirconia and less than 5 vol % of monoclinic zirconia. Measurements of bending strength, Vickers hardness and fracture toughness of the ceramic material were carried out according to the same procedure as Example 1. Results are listed in Table 5.

                                      TABLE 5     __________________________________________________________________________                       Average                            Average     Al.sub.2 O.sub.3               CeO.sub.2                   TiO.sub.2                       Grain                            Grain Vickers                                       Bending                                            Fracture                                                  Dispersion     Content   Content                   Content                       Size of                            Size of                                  Hardness                                       Strength                                            Toughness                                                  Ratio of     (vol %)   (mol %)                   (mol %)                       PSZ(μm)                            Al.sub.2 O.sub.3 (μm)                                  (GPa)                                       (MPa)                                            (MPa · m.sup.1/2)                                                  α-Al.sub.2 O.sub.3     __________________________________________________________________________                                                  (%)     Example 24           30  11  1   1.9  0.5   11.4 1130 12.9  3.9     Example 25           30  11  1   2.1  0.3   11.7 1180 12.1  4.7     Example 26           30  11  1   2.1  0.3   11.3 1160 12.5  4.6     Example 27           30  11  1   2.0  0.2   11.9 1220 11.8  5.0     __________________________________________________________________________      *PSZ = Partially Stabilized Zirconia 

What is claimed is:
 1. A process of making a zirconia based ceramic material comprising the steps of:preparing a mixture of a first constituent formed of a partially stabilized zirconia consisting essentially of 8 to 12 mol % of CeO₂, 0.05 to 4 mol % of TiO₂ and the balance of ZrO₂, and a second constituent formed of Al₂ O₃, a mixing ratio of said second constituent to said first constituent being such that said ceramic material essentially consists of 0.5 to 50 vol % of Al₂ O₃ and the balance of said partially stabilized zirconia; molding said mixture into a shape to form a green compact; and sintering said green compact in an oxidizing atmosphere under an atmospheric pressure to obtain said ceramic material.
 2. A process of making a zirconia based ceramic material as set forth in claim 1 comprising the step of performing a hot isostatic pressing (HIP) treatment to said ceramic material in an oxygen gas containing atmosphere after said sintering step.
 3. A process of making a zirconia based ceramic material as set forth in claim 1, wherein a powder of said partially stabilized zirconia as said first constituent is prepared by the sub-steps of:mixing a powder of a first partially stabilized zirconia containing CeO₂ and having an average particle size of 0.5 μm or less with a powder of TiO₂ having an average particle size of 0.5 μm or less to obtain a mixed powder; heating said mixed powder to obtain a calcined powder; and pulverizing said calcined powder.
 4. A process of making a zirconia based ceramic material as set forth in claim 1, wherein a powder of said partially stabilized zirconia as said first constituent is prepared by the sub-steps of:mixing a first aqueous solution of a zirconium salt with a second aqueous solution including cerium and titanium salts to obtain a mixed solution; hydrolyzing the mixed solution with an alkali aqueous solution to generate a precipitate; drying and heating said precipitate to obtain a first calcined powder; and pulverizing said first calcined powder.
 5. A process of making a zirconia based ceramic material as set forth in claim 1, wherein a powder of said partially stabilized zirconia as said first constituent is prepared by the sub-steps of:mixing an aqueous solution including zirconium and cerium salts with an organic solution of an alkoxide of titanium to obtain a mixed solution; hydrolyzing said mixed solution with an alkali aqueous solution to generate a precipitate; drying and heating said precipitate to obtain a first calcined powder; and pulverizing said first calcined powder.
 6. A process of making a zirconia based ceramic material as set forth in claim 1, wherein said second constituent is a powder of α-Al₂ O₃ having an average grain size of 0.5 μm or less.
 7. A process of making a zirconia based ceramic material as set forth in claim 1, wherein said second constituent is a powder of γ-Al₂ O₃ having a specific surface area of 100 m² /g or more.
 8. A process of making a zirconia based ceramic material as set forth in claim 1, wherein said mixture is prepared by the sub-steps of:mixing a powder of said partially stabilized zirconia as said first constituent with an aqueous solution of an aluminum salt as said second constituent to obtain a mixed solution; hydrolyzing said mixed solution with an alkali aqueous solution to generate a precipitate; drying and heating said precipitate to obtain a mixed powder; and pulverizing said mixed powder.
 9. A process of making a zirconia based ceramic material as set forth in claim 1, wherein said mixture is prepared by the sub-steps of:mixing a powder of said partially stabilized zirconia as said first constituent with an organic solution of an alkoxide of aluminum as said second constituent to obtain a mixed solution; hydrolyzing said mixed solution to generate a precipitate; drying and heating said precipitate to obtain a mixed powder; and pulverizing said mixed powder.
 10. A process of making a zirconia based ceramic material as set forth in claim 1, wherein said mixture is prepared by the sub-steps of:hydrolyzing a first aqueous solution of a zirconium salt with an alkali aqueous solution to prepare a sol solution of zirconia; mixing said sol solution with a second aqueous solution including cerium and titanium salts and a third aqueous solution of an aluminum salt to obtain a mixed solution; hydrolyzing said mixed solution with an alkali aqueous solution to generate a precipitate; drying and heating said precipitate to obtain a mixed powder; and pulverizing said mixed powder.
 11. A process of making a zirconia based ceramic material as set forth in claim 7, wherein said mixture is prepared by the sub-steps of:mixing a powder of said partially stabilized zirconia as said first constituent with said powder of γ-Al₂ O₃ to obtain a mixed powder; heating said mixed powder at a temperature of 1000° C. or more and less than a sintering temperature thereof to obtain a calcined powder; and pulverizing said calcined powder.
 12. A process of making a zirconia based ceramic material as set forth in claim 4, wherein said mixture is prepared by the sub-steps of:mixing the powder of said partially stabilized zirconia as said first constituent with an Al₂ O₃ powder as said second constituent to obtain a mixed powder; heating said mixed powder at a temperature of 1000° C. or more and less than a sintering temperature thereof to obtain a second calcined powder; and pulverizing said second calcined powder.
 13. A process of making a zirconia based ceramic material as set forth in claim 5, wherein said mixture is prepared by the sub-steps of:mixing the powder of said partially stabilized zirconia as said first constituent with an Al₂ O₃ powder as said second constituent to obtain a mixed powder; heating said mixed powder at a temperature of 1000° C. or more and less than a sintering temperature thereof to obtain a second calcined powder; and pulverizing said second calcined powder.
 14. A process of making a zirconia based ceramic material as set forth in claim 8 further including a treatment prior to said molding and sintering steps, said treatment comprising the steps of:heating said mixture at a temperature of 1000° C. or more and less than a sintering temperature thereof to obtain a calcined powder; and pulverizing said calcined powder.
 15. A process of making a zirconia based ceramic material as set forth in claim 9 further including a treatment prior to said molding and sintering steps, said treatment comprising the steps of:heating said mixture at a temperature of 1000° C. or more and less than a sintering temperature thereof to obtain a calcined powder; and pulverizing said calcined powder.
 16. A process of making a zirconia based ceramic material as set forth in claim 10 further including a treatment prior to said molding and sintering steps, said treatment comprising the steps of:heating said mixture at a temperature of 1000° C. or more and less than a sintering temperature thereof to obtain a calcined powder; and pulverizing said calcined powder. 